Conventionally, an electric charge is not measured directly, but rather from the electric potential which is developed across a known capacitance by the said charge or by a determined fraction thereof.
Such a measuring technique is difficult to adapt to sensing or detecting a rapid succession of electric charges, eg. the charges supplied by a cellular sensor such as an array of photodiodes where each cell supplies its charge in turn, by means such as charge transfer, transistor switching, electron beam scanning, etc.
Up to the present, as accurate a measurment capacitor as can be obtained has been charged either with the total charge to the measured or with a known fraction thereof, and then, after measuring the corresponding electric voltage, the capacitor has been discharged as completely as possible to make it ready to receive the next charge or fraction thereof for measurement.
The measurement capacitance is usually constituted either by the output capacitance of the set of sensors and/or by the input capacitance of the measuring circuit, or else by the integrating capacitance of an electronic integrator.
In either case, alternating between charging and discharging the capacitance is a limiting factor on the rate at which measurements can be peformed.
Further, if the integration method is used, it is necessary to discharge the integrating capacitance by applying a short circuit across its terminals, thereby greatly altering the parameters of the integrator feedback loop and thus compromising its stability. If an attempt is made to remedy this point, the set up time must be increased, thereby further reducing the maximum rate at which measurments can be made.